PTL 1 discloses an information system including a virtual storage apparatus 1000L and a virtual storage apparatus 1000R. The virtual storage apparatus 1000L provides a volume 3000LA to which the storage areas of its own HDDs 1030 are allocated, while the virtual storage apparatus 1000R provides a volume 3000RA to which the storage areas of HDDs 1030 in the virtual storage device itself are allocated. A same identifier is assigned to the volume 3000LA and the volume 3000RA. When receiving a write request for the volume 3000LA from a host, the virtual storage apparatus 1000L returns a response indicating that IO has succeeded to the host after confirming that both writing to the volume 3000LA and writing to the volume 3000RA of the virtual storage apparatus 1000R have been completed.